Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Typically, the working end of such torches consists of an elongated body having a generally cylindrical electrode located along the longitudinal axis of the working end. The working end of these torches often terminates in a nozzle that is separated from the electrode by a gas passage. The nozzle is electrically conductive and is insulated from the electrode so that an electrical potential difference can be established between the electrode and the nozzle for starting the torch.
To start the torch, one side of an electrical source, typically the cathode side, is connected to the electrode and the other side, typically the anode side, is connected to the nozzle through a switch and a resistor. The anode side is also connected in parallel to the workpiece. The starter circuit imposes a high voltage and high frequency potential difference across the electrode and nozzle, causing an electric arc to be established across the gas passage therebetween. This arc, commonly referred to as the pilot arc or non-transferred arc, has a relatively high frequency and high voltage but a relatively low current to avoid damaging the torch. Typically, the starter circuit in plasma arc torch control systems remains "on" until a pilot arc is detected, which can take a considerable amount of time after initiation of the starter circuit.
U.S. Pat. No. 4,782,210 to Nelson et al. attempts to minimize the starting voltage required for creating a pilot arc by using knurled ridges having sharp pointed edges on the side surface of the electrode. These ridges are oriented substantially along the flow lines of the plasma gas. The sharp edges enhance local electric fields to promote initiation of the pilot arc between the side surface of the electrode and the nozzle.
Once a pilot arc is created, the gas passing between the electrode and the nozzle is partially ionized. This ionized gas flows through thu nozzle and out of the working end of the torch. When the torch is moved close to a metallic workpiece, the ionized gas flowing from the working end toward the workpiece causes the arc to transfer off of the nozzle and on to the workpiece, which acts as an anode. This arc is referred to as the cutting arc, transferred arc, or main arc.
After the main arc is established, the switch connecting the potential source to the nozzle is opened and the power supplied to the torch is increased to create a main arc which is of a higher current than the pilot arc. The main arc is constricted in the nozzle as it extends from the electrode to the workpiece, thereby creating a high temperature plasma flow which heats the workpiece.
The electrode used in torches of the type described typically comprises a water-cooled elongate member composed of a material having high thermal conductivity, such as copper or a copper alloy. The discharge end of conventional electrodes is usually a flat smooth surface. An emissive element may be embedded in the flat surface for supporting the arc. The element is composed of a material which has a relatively low work function, defined in the art as the potential step, measured in electron volts, which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the element is capable of readily emitting electrons when an electrical potential is applied thereto. Preferred emissive element materials include hafnium, zirconium, or tungsten.
The electrode of a plasma arc torch is a consumable item, however, and thus it is desirable to increase electrode service life, particularly when the torch is used with an oxidizing or reactive gas such as oxygen or air. At least two factors contribute to the limits of the service life of the electrode. First, oxidizing gases tend to rapidly oxidize the copper in the electrode and as the copper oxidizes its work function falls. As a result, the oxidized copper which surrounds the emissive element begins to support the arc in preference to the element. When this happens, the copper oxide and the surrounding copper melt, resulting in the early destruction and failure of the electrode.
Second, the arc itself causes erosion of the electrode or emissive element at the arc attaching point. This is believed to be caused by the high temperature of the arc melting the material at the arc attachment point. Indeed, electrodes in plasma arc torches of the type described typically exhibit a concave erosion pit at the arc attachment point over time as the torch operates.
A pilot arc is often started on the cylindrical side surface of the electrode and then it migrates across the face of the electrode to the emissive element whereupon the arc is transferred to the workpiece. Testing has shown that, upon the first "start" of an electrode, the arc creates a trail as it moves across the electrode face to a point from which the arc is transferred to the workpiece. This point may not necessarily be in the exact center of the element.
In addition, for subsequent starts, the arc will frequently travel along the same trail and atransfer to the workpiece at the same, often uncentered point. This creates a problem because, as the electrode is operated, the concave erosion pit gradually increases in size. If the arc is not centered in the emissive element, the erosion pit caused by the arc will extend more quickly to the interface between the emissive material and the copper. When this happens, the arc will likely attach to the oxidized copper in preference to the emissive element, resulting in electrode damage or failure. Thus, it is desirable to ensure that the erosion pit begins at the exact center of the emissive element to maximize electrode life.
In U.S. Pat. No. 5,464,962 to Luo et al. and U.S. Pat. No. 5,726,414 to Kitahashi et al., the emissive surface of the electrode has a hole or recess preformed in the central region thereof. The predetermined recess in the Luo patent, the dimension of which is a function of the operating current of the torch, the diameter of the emissive element, and the plasma gas flow pattern, is said to reduce deposition of the high thermionic emissivity material on the nozzle during torch operation. The structure of the electrode in the Kitahashi patent is said to stabilize the main arc at a readily fixed cathodic point.
These electrode designs do not, however, ensure that the main arc attachment point always occurs in the central region of the emissive element. Instead, these patents are directed to minimizing erosion by the arc (Luo) or stabilizing the position of the arc (Kitahashi) when the arc has been attached to the center point of the electrode.
Accordingly, there is a need in the art for an electrode that facilitates arc starting and rapid transfer of the pilot arc to a main arc. Such an electrode would advantageously minimize deterioration by ensuring attachment of the main arc to the center of the emissive element. In this way, the concave erosion pit caused by the arc will be centered on the emissive element and the pit can deepen for the maximum amount of arc time before the main arc attaches itself to the adjoining copper material and destroys the electrode.